In a sodium-sulfur storage battery as shown in FIG. 2, which is a schematic longitudinally sectional view, an alpha-alumina ring 2 is jointed with solder glass to an upper end of a solid electrolyte tube 1 made from material such as beta-alumunina or beta-dash-alumina which is permeable to sodium ion. A negative auxiliary cover 3' and a positive cover 4 are thermocompressively jointed to the upper and lower surfaces of the alpha-alumina ring 2, respectively. A negative chamber inside the solid electrolyte 1 is filled with metal fiber 5 such as stainless steel, iron, Nichrome, aluminum or the like. A negative current terminal collector 10, which is made from copper coated by stainless steel and is welded to the negative cover 3, is inserted into the central part of said negative chamber. Further, sodium functioning as negative reactant 6 is filled by a vacuum impregnation manner through the top of the negative current terminal collector 10 into said chamber. The top end of the terminal collector 10 is sealingly closed by a vacuum welding to keep the vacuum condition. A positive chamber is formed between the solid electrolyte tube 1 and a battery housing 7 welded to the positive cover 4. The housing 7 is made from metal having sufficient resistance to chemical and electro-chemical attack by molten sodium and sodium polysulfide, and functions as a positive current collector. Positive electro-conductive conductive material 9 made of graphite felt, into which positive reactant 8, i.e., sulfur, is impregnated, is disposed in the positive chamber. A bottom cover 11 is welded to the battery housing 7 to seal the chamber in a vacuum condition.
When the sodium-sulfur storage battery having said structure is discharged at a battery operation temperature of about 350.degree. C., the negative reactant 6, i.e., sodium, in the solid electrolyte tube 1 is ionized and moves through the solid electrolyte tube 1 toward the positive electro-conductive material 9, and thus, reacts with the positive reactant 8, i.e., sulfur, in the positive electro-conductive material 9 to form discharge resultant subsutance Na.sub.2 S.sub.x. When said resultant substance becomes Na.sub.2 S.sub.2.7-3.0, the discharge finishes. In a subsequent charge, the discharge resultant subatance Na.sub.2 S.sub.x in the positive electroconductive material 9 changes and returns through Na.sub.2 S.sub.5 to the sulfur, and the sulfur ion returns through the solid electrolyte tube 1 to the negative chamber.
Although the sulfur is deposited sufficiently and substantially uniformly in the positive electroconductive material 9 at the charging operation, subsequent to the discharging operation, this deposited sulfur does not sufficiently diffuse and it sticks to the outer surface of the solid electrolyte tube 1 to form an electric resistance layer, which prevents or restricts the movement of the sodium ion to the negative chamber. Thus, a charging voltage increases to a predetermined or intended maximum value of about 3.0 V before the sufficient charging is performed, which results in such a problem that it becomes harder to obtain intended or regulated battery capacity as charge-discharge cycles increase.
Positive electroconductive material overcoming said problem is disclosed in Japanese laid-open patent publication 56-35374, in which needle punching is performed perpendicularly or crosswise to a direction of fiber to improve electroconductivity. However, in a mass production of the batteries employing the above positive electroconductive material, there is large deviation or difference between characteristics of the batteries, and manufacturing is expensive. Further, a volume of space is large with respect to the volume of the electroconductive material, so that there is a high electric resistance, resulting in such a problem that the battery capacity and discharging mean voltage decrease as the charge-discharge cycles increase.
It is an object of the present invention to provide an improved sodium-sulfur storage battery, of which the deviation of characteristics is decreased and manufacturing cost is also reduced by using positive electroconductive material having excellent productivity.